The heat exchanger made of an aluminum alloy includes a heat transfer tube, fins, and header pipes as the major components, and is manufactured by brazing. In the manufacturing process of the heat exchanger made of an aluminum alloy, the brazing sheet, on which an Al—Si alloy brazing material is cladded, has been utilized broadly. However, in recent years, products can be manufactured with low cost by applying a brazing filler composition, which is made of a mixture of: an Al—Si alloy powder or a Si powder; a flux; and a binder, on the surface of a heat transfer tube (extruded heat transfer tube) made of an extruded material without using the blazing sheet.
However, in the case of using the above-mentioned brazing filler composition, Si concentration increases on the surface of the heat transfer tube and decreases in the internal part of the tube since Si diffuses from the surface of the extruded heat transfer tube to the internal part due to heat during brazing, causing formation of electrical potential gradient, in which electrical potential is higher on the surface and lower in the internal part, in the heat transfer tube. Due to the electrical potential gradient formed in the heat transfer tube, the heat transfer tube corrodes and formation of pitting corrosion occurs, which causes the problems of refrigerant leakage or decreased strength.
As a countermeasure of these problems, a structure with improved corrosion resistance has been proposed. In this structure, electrical potential gradient, in which the electrical potential on the surface of the heat transfer tube is low and the electrical potential in the internal part is high, is formed by forming a Zn-diffusing layer on the surface of the heat transfer tube. The Zn-diffusing layer is formed by performing mixed application of a Zn-containing flux on the surface of the heat transfer tube with Si powder and the like.
The inventors of the present invention have proposed a tube for a heat exchanger in Patent Literature 1 (PTL 1). In the tube, a brazing coating, which includes Si powder whose application amount is 1-5 g/m2, and Zn-containing flux whose application amount is 5-20 g/m2, is formed on the outer surface of the extruded heat transfer tube, the outer surface being jointed to fins.
According to this proposal, the Si powder melts to be brazing liquid during brazing since the Si powder and the Zn-containing flux are mixed in. Thus, Zn in the flux diffuses uniformly in the brazing liquid and spreads over the surface of the tube uniformly. Diffusion rate of Zn is significantly higher in a liquid phase such as the brazing liquid than that in a solid phase. Thus, by having this configuration, Zn concentration can be kept uniform over the surface of the heat transfer tube. Because of this, a uniform sacrificial anode layer can be formed on the surface of the extruded heat transfer tube, and corrosion resistance of the extruded heat transfer tube for a heat exchanger can be improved.